Summary The long-term goal of this research project is to understand the mechanism that regulates energy balance. In the United States 35% of adults and 17% of youth are obese in 2011-2012. Obese youth and adults are developing type 2 diabetes at high rates and are at significant risk for life-threatening cardiovascular disease and cancer. Elucidation of the mechanism governing energy balance should provide an opportunity to develop novel effective and non-invasive therapeutic interventions. Brain-derived neurotrophic factor (BDNF) plays a critical role in regulating energy balance. Mutations in the genes for BDNF and its receptor TrkB lead to severe obesity in both mice and humans. Furthermore, the BDNF gene has been associated with human obesity in genome-wide association studies. However, we currently know much less about BDNF than leptin and MC4R with regard to the mechanisms underlying the role of these molecules in the control of energy balance. To help fill this knowledge gap, this research project tests the hypothesis that BDNF-expressing neurons in the paraventricular hypothalamus (PVHBDNF) integrate signals related to nutrients, energy store and reproduction, and project to both intra- and extra-hypothalamic targets to regulate appetite and energy expenditure. This hypothesis is based on the exciting findings we made during the prior grant period: (1) Deletion of the Bdnf gene in the PVH leads marked hyperphagia, impaired thermogenesis in brown adipose tissue (BAT) and severe obesity; (2) PVHBDNF neurons are polysynaptically connected to BAT through sympathetic preganglionic neurons in the spinal cord to stimulate adaptive thermogenesis; (3) PVHBDNF neurons are also polysynaptically connected to white adipose tissue (WAT), suggesting their role in induction of beige adipocytes; (4) PVHBDNF neurons project densely to the arcuate nucleus of the hypothalamus (ARH), dorsomedial hypothalamus (DMH), ventral region of the lateral parabrachial nucleus (LPBN) and nucleus of the solitary tract (NTS) in addition to the spinal cord; (5) PVHBDNF neurons receive inputs from the ARH, DMH and rostral periventricular region of the 3rd ventricle (RP3V). As PVHMC4R neurons receive input from the ARH and project densely to the central LPBN, NTS and dorsal motor nucleus of the vagus, our findings indicate that PVHBDNF neurons and PVHMC4R neurons have distinct projection targets and input sources, suggesting that these two groups of neurons regulate energy balance in different contexts. Therefore, it is imperative to delineate neural circuits through which BDNF suppresses appetite and promotes thermogenesis in order to fully understand the central control of energy balance. We propose to test our hypothesis in three specific aims. Aim 1 is to identify which PVHBDNF projection regulates energy balance; Aim 2 is to determine the identity and function of afferent neurons to PVHBDNF neurons; Aim 3 is to investigate the role of PVHBDNF neurons in induction of beige adipocytes within WAT. Findings from the proposed studies will uncover novel neural circuits that regulate appetite and adaptive thermogenesis. !